How long can a package of ground beef (that has been opened but stored in a zip lock bag) stay in the refrigerator before needing to throw it

Ground beef's particularly susceptible to spoilage because of the surface area - firstly it comes into contact with the grinding surfaces and then is exposed to air & moisture as it sits. I'd toss.

A week is a long time to keep ground beef even refrigerated.

My Snidely Whiplash scheme is to substitute SPAM in what I refer to as Tiki Tacos. That stuff never goes bad! And if you disguise it well enough people actually love it.

Probably not.

How does it look/smell? Anything "off" look or smell wise is a dead give away (pun intended). Never more than with meat -- "when in doubt, throw it out". Saving $X on meat by not replacing is never worth the illness/doctors' costs that might ensue. And, if budget is an issue, use canned black beans to replace the meat (cook up the same way you would the meat -- seasonings and all) -- you may discover a new preference! (I do this all the time -- replacing ground beef with black beans in Mexican food -- not only does my family not complain, most of the time they don't even notice. Healthier and cheaper -- how great is that??!)/